Vehicle manufacturers have employed active and passive methods to reduce engine noise within the passenger compartment. Noise frequently emanates from the engine, travels through the air induction system and passes out of the mouth of the air intake into the passenger compartment. Efforts have been made to reduce the amount of engine noise traveling through the air induction system. These efforts include the use of both passive devices, such as expansion chambers, and active devices involving anti-noise generators.
Active noise control systems use a speaker that outputs a generated sound, or “antinoise,” through a speaker to attenuate engine noise. The generated anti-noise is out of phase with the engine noise and combines with the engine noise to reduce the overall noise level from the engine. Generally, this sound is generated in proximity to the air induction system. The resultant mix of engine noise and anti-noise is captured and measured by the microphone, and the output of the microphone is used to determine what the generated anti-noise should be. In short, active noise control is conducted via a feedback loop.
Because the microphone must operate in a hostile environment over many years, the output of the microphone for a given engine noise pattern may change. The engine noise level itself, however, does not change over time. Dirt, humidity, and other environmental factors may all cause the microphone to lose sensitivity and therefore output an incorrect signal for generating the anti-noise. Other system components, such as speakers that output the anti-noise, may also be adversely affected by environmental factors.
There is a desire for a method and system that is not affected by changes in the microphone in generating an accurate anti-noise signal for effective noise cancellation.